1. Field of the Invention
The present invention relates to a water-permeable humidifier for fuel cell which uses, for example, hollow fiber membranes.
2. Description of Related Art
A PEM (Polymer Electrolytic Membrane) type fuel cell, which can be mounted in a fuel cell vehicle and the like, is formed by laminating an membrane electrode assembly comprising an anode and a cathode on each side of a solid polymer electrolytic membrane, and a separator which provides gas paths for supplying reactive gas on each side of the membrane electrode assembly and supports the membrane electrode assembly from both sides.
In this PEM-type fuel cell, hydrogen gas is supplied as fuel supply gas to the anode, and oxygen or air is supplied as oxidizing agent supply gas to the cathode, and the chemical energy generated by the oxidation-reduction reaction of the fuel supply gas is extracted as direct electrical energy.
That is, electrical energy can be extracted by a series of electrochemical reactions when the hydrogen gas is ionized on the anode side and moves through the solid polymer electrolytic membrane, and the electrons pass through an external load to the cathode side, reacting with the oxygen and generating water.
In this fuel cell, when the solid polymer electrolytic membrane dries, the ion transmittancy rate decreases and the energy conversion rate is reduced. Consequently, water must be supplied to the solid polymer electrolytic membrane in order to maintain good ion transmission.
To achieve this, this type of fuel cell comprises a humidifier which humidifies the fuel supply gas and oxidizing agent supply gas, and supplies water to the solid polymer electrolytic membrane, thereby maintaining good reaction.
A conventional example of this type of humidifier, disclosed in Japanese Patent Application No. 8-273687, is a water-permeable humidifier comprising a hollow fiber membrane which allows steam vapor to pass in the direction of the thickness of a membrane.
FIG. 16 is a diagram showing the constitution of a fuel cell system comprising a conventional humidifier. Oxidizing agent supply gas comprises outside air, and is pressurized by a supercharger 81 and supplied via an oxidizing agent supply gas pipe 82 to a humidifier for oxidizing agent 80A. The humidifier for oxidizing agent 80A humidifies the oxidizing agent supply gas and supplies it to the cathode of a fuel cell (hereinafter “stack”) 83. After the oxygen supplied in air to the cathode has been used as the oxidizing agent, it is exhausted as off-gas from the stack 83. The off-gas contains water which was generated at the time of the reaction in the stack 83, and is fed from the stack 83 via an off-gas pipe 84 to the humidifier for oxidizing agent 80A. The humidifier for oxidizing agent 80A transfers steam vapor in the off-gas to the oxidizing agent supply gas. Thereafter, the off-gas is exhausted.
Fuel supply gas comprising hydrogen gas is supplied via a gas pipe for fuel supply 85 to a humidifier for fuel 80B, which humidifies the hydrogen gas and supplies it to the anode of the stack 83. Part of the hydrogen gas supplied to the anode is used as fuel, and is applied in the oxidation-reduction reaction. After part of the hydrogen gas has been applied in the reaction, it is exhausted from the stack 83 as off-gas.
However, the solid polymer electrolytic membrane allows steam vapor to pass through from the side of the membrane where water density is high to the side where water density is low as a result of ion hydration. As described above, the off-gas flowing on the cathode side contains water which was generated at the time of reaction, and consequently has a higher water density than the off-gas flowing on the anode side, but the ion hydration causes the water in the off-gas flowing on the cathode side to become steam vapor and pass through the solid polymer electrolytic membrane, and is dispersed in the off-gas flowing on the anode side. Therefore, the off-gas on the anode side also contains water.
The anode-side off-gas containing water is fed from the stack 83 via a pipe for off-gas 86 to the humidifier for fuel 80B. The humidifier for fuel 80B transfers the steam vapor in the off-gas to the oxidizing agent supply gas. Thereafter, the off-gas is exhausted.
FIG. 17 shows the humidifier for oxidizing agent 80A and the humidifier for fuel 80B (hereinafter jointly referred to as humidifier 80 unless there is a need to distinguish them). The humidifier 80 comprises a plurality of humidifying units 91, and an entrance head 92 and an exit head 93 which join the humidifying units 91 in parallel. The humidifying units 91 comprise a great number of tube-like porous hollow fiber membranes 95, which are bundled together inside a cylindrical housing 94. The porous hollow fiber membrane is consisted of steam vapor-permeable membranes (water-permeable membranes). Partitioning members 96 tie both ends of the hollow fiber membranes 95, and achieve an airtight seal between the outer surfaces of the hollow fiber membranes 95, and between the outer surfaces of the hollow fiber membranes 95 and the housings 94. One end of the housings 94 is connected to the entrance head 92, and the other end is connected to the exit head 93. Gas entrances 97a and gas exits 97b are provided in the outer peripheral section of the housings 94 further inward from the partitioning members 96. The gas entrances 97a of the housings 94 are connected together via an unillustrated connection path, provided outside the housings 94. Similarly, the gas exits 97b are connected together via an unillustrated connection path, provided outside the housings 94.
In the humidifier 80, reactive gas is supplied from the gas entrance hole 97a in the housing 94 of each humidifying unit 91, passing between the hollow fiber membranes 95 of the housings 94 and exiting from the gas exit 97b. On the other hand, off-gas is supplied to the entrance head 92, from the entrance head 92 to the housing 94 of the humidifying unit 91 and into the hollow section of the hollow fiber membrane 95, passing through the hollow section and from the other side of the housing 94 into the exit head 93, and exiting from the exit head 93.
The hollow fiber membranes 95 have countless capillary tube sections running parallel to the diameter; steam vapor in the off-gas, which is fed into the hollow sections of the hollow fiber membranes 95, condenses in the capillary tube sections and moves to the outer peripheral side, where it is transferred by evaporation to reactive gas. That is, the humidifier 80 transfers the water in the off-gas to the reactive gas, thereby humidifying the reactive gas.
However, in the conventional humidifier 80, steam vapor in the off-gas condenses in the entrance head 92, causing the following problems.
When condensation had seeped into the hollow section of the hollow fiber membrane 95, the condensation cannot pass through the hollow fiber membrane 95 and flows through the hollow section. As a consequence, the steam vapor is exhausted without being recovered, reducing the water recovery rate. When the water recovery rate decreases, the humidification capability of the humidifier decreases.
Also, if the fuel cell is excessively humidified, a problem may occur in that gas flow paths in the fuel cell become closed by the excess water and the output of the fuel cell decreases.
Furthermore, when condensation accumulates in the entrance head 92 and the water level rises higher than the minimum position of the hollow fiber membrane 95 in the humidifying unit 91, the condensation closes up the entrance side of the hollow fiber membrane 95, reducing the flow area for the off-gas and increasing pressure loss. Further, the decrease in the flow area of the off-gas leads to a reduction in the water recovery rate, and a consequent decline in the humidification capability. The same problems arise when condensation accumulates in the exit head 93.